1. Field of the Invention
The present invention relates to a color liquid crystal display device used in a portable information device such as a mobile phone or an electronic notebook, a personal computer, or the like. In particular, the invention relates to a semi-transmissive type color liquid crystal display device serving both as a reflection type color liquid crystal display device and a transmission type color liquid crystal display device, a method of manufacturing the same, and a method of manufacturing a color filter substrate.
2. Description of the Related Art
As a color liquid crystal display device used for a display device in a mobile phone, a portable information device, or the like, three types: a transmission type, a reflection type, and a semi-transmissive type of color liquid crystal display devices are used. Hereinafter, brief description will be made of the three types of conventional color liquid crystal display devices with reference to the accompanying drawings. FIG. 1 is a sectional view of a conventional transmission type color liquid crystal display device. A light shielding film 4 and color filters 3 are formed on the surface of a lower transparent substrate 1A. The color filter 3 includes colored portions of three primary light colors: red (R), green (G), and blue (B) according to a pattern. That is, the colored portions of red, green, and blue are formed in the color filter 3 in an arbitrary pattern such as a stripe or a mosaic. The light shielding film 4 is provided between the colored portions as appropriate. A transparent flattening film 5 is provided on the surfaces of the light shielding film 4 and the color filters 3, and on top of the flattening film a transparent electrode 2A is formed in an arbitrary pattern. In the case of a passive type color liquid crystal display device, the transparent electrode 2A is formed in such a pattern that the transparent electrode crosses colored layers 3R, 3G, and 3B of the color filters. That is, the transparent electrode 2A is formed as a common line. In the case of an active type color liquid crystal display device, the transparent electrode 2A is not patterned, and may be left as it is in an electrode shape after performing film formation by using a film formation mask. Those layers are collectively referred to as a color filter substrate 6.
As shown in the drawing, a gap between the color filter substrate 6 and a transparent substrate 1B as a counter substrate is uniformly maintained by sealing members 7 and spacers 9, and a liquid crystal 10 is filled in a space defined by the color filter substrate 6 and the transparent substrate 1B. A display panel thus structured is sandwiched between a pair of deflection plates 12 and 13. In addition, orientation films (not shown) are provided on the surfaces of transparent electrodes (2A and 2B) (see Tatsuo Uchida (Nov. 1, 1994) “New Technology of Liquid Crystal Display”, p. 167–174, published by Kogyochosakai Publishing Co., Ltd., for instance).
FIG. 2 is a sectional view of a conventional reflection type color liquid crystal display device. The aforementioned description of the transmission type color liquid crystal display device of FIG. 1 will be omitted. As shown in FIG. 2, a metallic reflective film 11 for reflecting outside light is provided between the transparent substrate 1A and the color filters 3. Therefore, the deflection plate 12 becomes unnecessary unlike the transmission type display device shown in FIG. 1. In this case, the deflection plate 13 is often provided with a quarter-wave plate for returning phase-shifted light due to reflection at the metallic reflective film 11 and a layer having a scattering function for glare prevention of regularly reflected light at the metallic reflective film 11 (see Tatsuo Uchida (Nov. 1, 1994) “New Technology of Liquid Crystal Display”, p. 167–174, published by Kogyochosakai Publishing Co., Ltd., for instance).
FIG. 3 is a sectional view of a conventional semi-transmissive type color liquid crystal display device. The above-mentioned description regarding the color liquid crystal display devices will be omitted. As shown in FIG. 3, the metallic reflective film 11 formed between the transparent substrate 1A and color filters 3 is partially removed. Therefore, both functions of a reflection part and a transmission part are achieved in one pixel element. Further, the deflection plate 12 is provided on the outer side surface of the transparent substrate 1A (see JP 11-52366 A (pages 2–4, FIG. 1), for instance).
Subsequently, brief description will be made of a method of manufacturing the conventional semi-transmissive type color liquid crystal display device. First of all, as shown in FIG. 4A, the metallic reflective film 11 is formed on the surface of the transparent substrate 1A by a vacuum film formation method such as sputtering or a vacuum deposition method so as to have the thickness at a level in which no light is transmitted therethrough. To obtain sufficient light shielding property, the thickness of the metallic reflective film 11 needs to be 0.10 μm or more. In the case where the metallic reflective film 11 is made of aluminum or an aluminum alloy, the thickness thereof is set to approximately 0.125 μm in general. In the case where the metallic reflective film 11 is made of silver or a silver alloy, the thickness thereof is generally set to about 0.10 μm. Next, as shown in FIG. 4B, the metallic reflective film 11 is patterned by a photolithography method. Patterning is conducted such that the reflection part and the transmission part are established in each pixel element in a display screen of the display panel. In FIG. 4B, a part where the metallic reflective film 11 is left corresponds to the reflection part, and a part where the metallic reflective film 11 is removed becomes the transmission part. The proportion of the reflection part and the transmission part can be set arbitrarily through patterning.
Then, as shown in FIG. 4C, the patterned light shielding film 4 is formed. To obtain the light shielding film 4, the entire surface of the transparent substrate 1A is applied with a liquid photoresist containing black pigments, and thereafter the resist is patterned into a desired shape by the photolithography method. Typically, this is called a black matrix as being formed in a matrix. In these days, to increase the reflection light amount of the color liquid crystal display device, the light shielding film 4 is formed into a stripe shape (black stripe), or sometimes the light shielding film 4 is provided only in a frame part around the color liquid crystal display device and not provided in its display area.
Subsequently, as shown in FIG. 4D, the colored portions 3R constituting the color filters are formed. To obtain red color filters, the entire surface of the transparent substrate 1A is applied with a liquid photoresist containing red pigments, and the resist is then patterned into a desired shape by the photolithography method. The colored portions are usually formed in a stripe shape along the matrix of the light shielding film 4. In a similar manner, the colored layers 3G (green) and 3B (blue) are sequentially formed to obtain such a shape as shown in FIG. 4E. Here, there is almost no difference between the thickness of the respective colored layers 3R, 3G, and 3B obtained through the application of the liquid color resist on the metallic reflective film 11 (that is, at the reflection part) and the thickness of the respective colored layers 3R, 3G, and 3B on the transparent substrate 1A where the metallic reflective film 11 is removed (that is, at the transmission part). This is because the metallic reflective film 11 is a thin film having the thickness of approximately 0.10 μm, and the liquid photoresist is turned into a film along the surface shape of the substrate 1A.
Next, as shown in FIG. 4F, the flattening film 5 made of a transparent resin is formed on the surfaces of the color filters 3 in which those colored portions are formed. In general, the flattening film 5 is formed through application of a liquid material by use of a spinner. As shown in FIG. 4G, the transparent electrode 2A is sequentially formed on the surface of the flattening film 5. The flattening film 5 has adhesiveness with respect to the transparent electrode 2A, resistance to patterning, and the like. The transparent electrode 2A is typically formed by a sputtering method so as to have desired thickness and resistance value characteristics. In general, a conductive material containing an incompletely oxidized alloy of indium (In) and tin (Sn) is used for forming the transparent electrode 2A.
In this way, the substantially planer color filter substrate 6 shown in FIG. 4H is completed. As described above, by using the color filter substrate 6, the conventional semi-transmissive type color liquid crystal display device is formed. Hereinafter, brief description will be made of a method of manufacturing a liquid crystal display device. Orientation films provided on the surfaces of the color filter substrate 6 and a counter substrate 8 are commonly formed by an offset printing method. The spacers 9 provided between the color filter substrate 6 and the counter substrate 8 are uniformly distributed by a dispersion method. The sealing members 7 are formed by a screen printing method in usual cases. The color filter substrate 6 and the counter substrate 8 are glued together, and after that the liquid crystal 10 is filled in the space corresponding to the gap between the color filter substrate 6 and the counter substrate 8.
Further, recently, there has come along a technique for improving color reproducibility at the time of light transmission by increasing the thickness of the color filter at the transmission part as compared to the thickness of the color filter at the reflection part (see JP 2002-303861 A (pages 2–4, FIG. 1), for instance). Brief description will be made of the technique by referring to FIGS. 5A to 5G and 6. First, as shown in FIG. 5A, the surface of the transparent substrate 1A is applied with a photosensitive transparent resin, and resin layers 14 are formed by the photolithography method. To obtain the resin layers 14, a considerably complicated method is adopted as follows. That is, a positive type photoresist having a property of melt flow upon post-baking is formed in a desired pattern, and then projections and depressions are formed on the layer surface by post-baking. After that, application of the positive type photoresist is conducted again to cover the projections and depressions. Thus, the photolithography method is used twice by way of double photoresist applications (see JP 6-11711 A (pages 2 and 3, FIG. 4), for instance). As in the pattern of the metallic reflective film 11 described in the case of the above-mentioned conventional semi-transmissive type color liquid crystal display device, the resin layers 14 have a shape in which the reflection part and the transmission part are established in one pixel element. Next, the metallic reflective film 11 is formed on the entire surface of the transparent substrate 1A by sputtering or the like, and then the film is patterned by the photolithography method to have such a pattern that the metallic reflective film 11 overlaps the surfaces of the resin layers 14. After that, as shown in FIG. 5C, the light shielding film 4 is formed. To obtain the colored portions of the color filters, red colored portions 3R and 3R2, green colored portions 3G and 3G2, and blue colored portions 3B and 3B2 are formed by employing the photolithography method six times with negative type color resists for the reflection part and the transmission part, separately (FIGS. 5D to 5F). As described above, by using the color filter substrate 6, another conventional semi-transmissive type color liquid crystal display device shown in FIG. 6 is obtained.
As another method of forming a color filter, there is also disclosed in JP 2002-303861 A (pages 2–4, FIG. 1) a method of forming color filters at the reflection part and the transmission part at the same time. However, no description is given of a specific method of doubling the thickness of the color filter at the transmission part as compared to the thickness of the color filter at the reflection part, and it is merely described that “application is performed so as to have the thickness” in the transmission area twice as large as the thickness in the reflection area. The inventors of the present invention can understand that the thickness at the reflection part and that on the transmission part are changed in employing the photolithography method six times as described above, but conceive that it is not easy to understand the method of “forming color filters at the reflection part and the transmission part at the same time”, which offers no specific description.
As described above while referring to FIGS. 5A to 5G and 6, in order to achieve a satisfactory color balance upon light reflection and transmission, the semi-transmissive type color liquid crystal display device having the thickness at the transmission part larger than the thickness at the reflection part has been devised. However, such a semi-transmissive type color liquid crystal display device has the following problem. That is, since the colored portions of the respective colors in the color filters are formed by performing a photolithography step one time each, the color filters 3 having substantially the same thickness are formed in any of the display areas. Accordingly, levels of color density and brightness achieved by the respective color filters are the same in the entirety of the display area. In addition, from the relationship regarding productivity of the reflective film 11, the thickness thereof is set to about 0.10 μm, and the colored portions are formed on the reflective film 11 through a spinner method by using the liquid color resist, so that there is almost no difference in film thickness at the reflection part and the transmission part. Therefore, there is also almost no difference in color density and brightness achieved by the color filters at the entirety of the display area. In other words, if the film thickness of the color filter is increased to enhance display color density of the semi-transmissive type color liquid crystal display device, transmittance of the color filter is lowered, which leads to a problem of decrease in brightness at the transmission part and the reflection part. Conversely, if the film thickness of the color filter is decreased with the emphasis on brightness at the display image, there is a problem in that the display color density cannot sufficiently be obtained. Moreover, at the reflection part, outside light is transmitted through the color filter layer and thereafter reflected at the metallic reflective film, and the reflected light returns while being transmitted through the color filter layer again. Thus, the incident light amount is drastically reduced because of the light transmission through the color filter layer twice. For this reason, there is a problem in that visibility of the color liquid crystal display device deteriorates.
Further, in addition to this structure, it is conceivable to provide a structure in which after forming color filters having small thickness, color filters are further formed only at the transmission part 12. However, in this structure, the number of times to perform the photolithography step for the color filters needs to be doubled, i.e., six times, resulting in lowering productivity and increasing industrial and economic burdens such as increase in the number of defects. In the case of the color filters used for the color liquid crystal display device having a refined color filter shape, since high-precision alignment by the above-mentioned photolithography method is required, it is necessary to perform an extremely troublesome manufacturing step. At the same time, productivity severely deteriorates and also yield is lowered so that industrial and economic problems are unavoidable.
Note that, the inventors of the present invention point out that the extremely troublesome method of forming the resin layers 14 is a prerequisite also for the method of setting the thickness of the color filter at the transmission part larger than that of the color filter at the reflection part through the simultaneous formation of the color filters, which is disclosed in JP 2002-303861 A (pages 2–4, FIG. 1).